watching with interest as my DS (2yo2m) has started to hit out and push - mostly his "best friend", but also I've noticed at the playground recently occasionally with other children.
I always do the "no, stop, we don't hit, say sorry" etc... I've wondered whether it's because he's not really talking yet; words are increasing, but it seems quite slow, and I wonder whether he is frustrated that he can't communicate as he would like?
The situation with his friend is interesting as I think the friend IS a bit of a wind-up merchant... always wants to have the toy DS has, always makes a big fuss when he doesn't get what he wants, will cry even at a small push etc. DS doesn't take from others, but he is VERY possessive about what he is playing with, and it takes a LOT of discussion to share and give over toys etc to others (know that's normal at this age).
But I empathise with your situation as I get on really well with the mum, but it is a bit wearing, and I have started to reduce the time we spend together as I find I can't relax as I need to be very alert to DS around her son.
I know it's a stage, but it's dull!